The invention relates to block copolymers and more especially to thermoplastic elastomer block copolymers.
Thermoplastic elastomers are known. These polymers present interesting mechanical properties (elastomeric) properties, with interesting handling and shaping properties (thermoplastic). Whereas classical elastomers usually require a chemical cross linking step, usually not reversible, before or after they have been shaped or applied onto a support, thermoplastic elastomer are cross linked according to a physical phenomenon that is reversible by heating. This property allows to obtain elastomeric compounds without any chemical cross linking step that may be difficult to carry out on some supports, or that would require complex formulations (two or more components formulations, or formulations comprising a catalyst being active only upon particular conditions such as dryness). Thermoplastic Elastomers may be shaped (extrusion, injection . . . ) or applied onto a support in a melted form. They are used for various purposes in various fields, including soft touch plastics for consumer goods or car interior, structural or mechanical articles, adhesives. They may also be used as additives in thermoplastics or in compositions used in building industry, to modify the mechanical properties of said thermoplastics or compositions.
Triblock copolymers comprising two styrene stiff side blocks, and a soft central block comprising units deriving from diene monomers have been known and used for years. Examples include Styrene-Butadiene-Styrene (SBS) block copolymers, Styrene-(Ethylene Butadiene)-Styrene (SEBS) block copolymers, and hydrogenated ones. Styrene blocks are stiff blocks forming a stiff phase dispersed in a soft phase of the central block (microphase seperation). Without intending to be bound to any theory, and as a simplified view, it is believed that the dispersion of the stiff phase provides cohesion between the polymer chains, the other phase presenting elastomeric properties. This phenomenon may be referred to as a cross linking according to a physical phenomenon. Hence, a microphase separation of the blocks is believed to be a key factor. When heating, the stiff phase melts and the copolymers can be handled in a liquid form. Considerations on microphase separation of block copolymers are provided in Frank S. Bates and Glenn H. Fredrickson, Physics Today, February 1999 issue, pages 32-38.
The triblock copolymers mentioned above present some drawbacks. First, the central block sometime gets yellow with age. Second, for some uses, for example uses as adhesives or uses wherein compatibilization with other compounds is needed, some other functions have to be added, such as anhydride, carboxylic acid, ester, epoxide functions by copolymerizing additional comonomers. To avoid yellowing, and to address needs for some uses, block deriving from esters of acrylic acid or of methacrylic acid could be a solution.
Further, the above mentioned triblock copolymers are usually prepared by an anionic polymerization process, with optionally a hydrogenation step. Anionic polymerization processes are usually considered as expensive over free-radical polymerization processes. Hence, there is a need for block copolymers that may be prepared by free-radicals polymerization, and allow phase separation of the blocks as explained above, or, more simply, that present thermoplastic elastomers properties. On another hand, obtaining block copolymers polymers with high and controlled molecular weight is easier with an anionic polymerization process than with a free-radical polymerization process: it may be difficult to obtain high molecular weight block copolymers, with a controlled structure and with controlled molecular weights by a free-radical polymerization process. At the time being, no solution has been found to the problem of proposing a block copolymer comprising a soft block deriving mainly from esters of acrylic acid or of methacrylic acid, with reasonable cost. As an aside, it is known that the higher the molecular weight of the blocks is, the easier the phase separation is.
Applicant has found new block copolymers which address some of the needs or problems listed above. The block copolymers comprise a soft block comprising esters of acrylic acid or of methacrylic acid, which prevents or postpones yellowing, and provide the block copolymers with interesting adhesion or compatibilization properties. The block copolymer that the applicant has found may be prepared by a free-radical polymerization process. Hence, the invention also relates to a process for making the block copolymer.
The invention relates to a block copolymer, being a diblock (block A)-(bloc B) copolymer, a triblock (block A)-(block B)-(block A), or a triblock (block A)-(block B)-(block C) copolymer,
block A being a stiff block, comprising at least 55% of repetitive units deriving from a stiff monomer selected from the group consisting of styrene, isobornyl acrylate, and isobornyl methacrylate,
block B being a soft block, comprising at least 55% of repetitive units deriving from a soft monomer selected from the group consisting of esters of acrylic acid, esters of methacrylic acid, and mixtures thereof,
block C being a stiff block, comprising repetitive units deriving from a stiff monomer selected from the group consisting of styrene, isobornyl acrylate, and isobornylmethacrylate,
wherein:
block A comprises at least 6% by weight of units deriving from a block A segregation monomer selected from the group consisting of acrylic acid, methacrylic acid, acrylonitrile, methacrylonitrile, 2-vinylpyridine, and 4-vinylpyridine, or
block B comprises at least 6% by weight of units deriving from a block B segregation monomer selected from the group consisting of acrylic acid, methacrylic acid, acrylonitrile, methacrylonitrile, and vinylpyrrolidones, and
the block copolymer has a molecular weight of more than 10,000 g/mol.
The block copolymers according to the invention may be in a solid or dried form. They may also be dissolved in an organic solvent. They may also be in the form of a latex dispersion in an aqueous medium. Such dispersions or solutions are usually intended to be applied onto a surface, and then dried, to make a film or a coating. In a dried form, the block copolymer according to the invention presents elastomeric properties. It presents also thermoplastic properties.
In a second aspect, the invention relates to a process for making the block copolymer defined above, said process being a sequenced living (and controlled) free-radical polymerization process, preferably involving using a transfer agent comprising a group of formula xe2x80x94Sxe2x80x94C(S)xe2x80x94Xxe2x80x94, xe2x80x94Sxe2x80x94C(S)xe2x80x94Sxe2x80x94, or xe2x80x94Sxe2x80x94P(S)xe2x80x94Xxe2x80x94, or xe2x80x94Sxe2x80x94P(S)xe2x80x94Sxe2x80x94, wherein X is an atom different from sulfur.
In a third aspect, the invention relates to the use of the block copolymer defined above as an adhesive, to its use in an adhesive composition, and to adhesive compositions or products comprising it.